Unusual void galaxy DDO68: implications of the HST resolved photometry

TL;DR

This study uses HST resolved photometry to analyze the stellar populations and star formation history of the unusual, metal-poor dwarf galaxy DDO68, revealing its past quenching, recent burst, and possible merger activity.

Contribution

First detailed analysis of DDO68's resolved stellar populations using HST data, revealing its star formation history and merger evidence.

Findings

Approximately 60% of stars formed 12-14 Gyr ago.

Recent star formation burst occurred about 300 Myr ago.

Majority of stars have very low metallicity (Z = Zsun/50 to Zsun/20).

Abstract

DDO68 (UGC5340) is an unusual dwarf galaxy with extremely low gas metallicity (12+log(O/H) = 7.14) residing in the nearby Lynx-Cancer void. Despite its apparent isolation, it shows both optical and HI morphological evidence for strong tidal disturbance. Here, we study the resolved stellar populations of DDO68 using deep images from the HST archive. We determined a distance of 12.75+-0.41 Mpc using the tip of the red giant branch (TRGB). The star formation history reconstruction reveals that about 60 per cent of stars formed during the initial period of star formation, about 12-14 Gyr ago. During the next 10 Gyr DDO68 was in the quenched state, with only slight traces of star formation. The onset of the most recent burst of star formation occurred about 300 Myr ago. We find that young populations with ages of several million to a few hundred million years are widely spread across various parts of DDO68, indicating an intense star formation episode with a high mean rate of 0.15 Msun/yr. A major fraction of the visible stars in the whole system (~80 per cent) have low metallicities: Z = Zsun/50 - Zsun/20. The properties of the northern periphery of DDO68 can be explained by an ongoing burst of star formation induced by the minor merger of a small, gas-rich, extremely metal-poor galaxy with a more typical dwarf galaxy. The current TRGB-based distance of DDO68 implies a total negative peculiar velocity of ~500 km/s.